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Bilaterians
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lecture notes on bilaterians from first year (part of bilaterians, molluscs, and arthropods)
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Lecture 1: Bilaterians IDivided into protostomia and deuterostomia.
Bilaterians - Deuterostomia
These have only one plane of symmetry.
Bilateria: locomotion
Diploblasts have no locomotion along environmental gradients. Bilaterians have locomotion across
environmental gradients. This gives it an advantage in finding food, mates and avoiding predators.
The cephalization of bilaterians means that it has all of its sensory organs concentrated at the front
of its head. This creates a polarity since there's a concentration of nervous system.
Locomotion across gradients causes bilateral symmetry. Cnidarians which move across gradients
evolve a degree of bilateral symmetry.
Some species that have secondary radial symmetry are:
Echinoderm
Annelid
Bryozoa
Sessile bilaterians revert back to radial symmetry.
For locomotion, it requires muscles which contract and relax. Also needs a skeleton of some sort
(endo skeleton in vertebrates and exoskeleton in arthropods)
Bilateria have a hydrostatic skeleton that allows it to move by peristaltic.
Hydrostatic skeleton is formed when an enclosed volume of fluid is used to support and counteract
muscles. Its well suited for aquatic and damp terrestrial habitats.
Body cavities
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